Page 377 - Handbook of Energy Engineering Calculations
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environmental regulatory authorities, and (2) to recharge the geothermal
resource.
One recent trend in the industry is to collect noncondensable gases (NCGs)
purged from the condenser and reinject them along with the brine. Older
plants use pollution-abatement devices to treat NCGs, then release them to
the atmosphere. Reinjection of NCGs with brine lowers operating costs and
reduces gaseous emissions to near zero.
Major improvements in flashed-steam plants over the past decade centered
around are: (1) improving efficiency through a dual-flash process and (2)
developing improved water treatment processes to control scaling caused by
brines. The pressure of the liquid brine stream remaining after the first flash
is further reduced in a secondary chamber to generate more steam. This two-
stage process can generate 20 to 30 percent more power than single-flash
systems.
Most of the recent improvements in binary-cycle plants have been made by
applying new working fluids. The thermodynamic and transport properties of
these fluids can improve cycle efficiency and reduce the size and cost of heat-
transfer equipment.
To illustrate: By using ammonia rather than the more common isobutane or
isopentane, capital cost can be reduced by 20 to 30 percent. It is also possible
to improve the conversion efficiency by using mixtures of working fluids,
which in turn reduces the required brine flow rate for a given power output.
A flashed-steam cycle will be tentatively chosen for this installation
because the brine free-flows at 450°F (232°C), which is higher than the cutoff
temperature of 350°F (177°C) for binary systems. An actual plant (Fig. 5),
operating with these parameters uses two flashes. The first flash produces
2
623,000 lb/h (283,182 kg/h) of steam at 100 lb/in (gage) (689 kPa). In the
second flash an additional 262,000 lb/h (117,900 kg/h) of steam at 10 lb/in 2
(gage) (68.9 kPa) is produced.